"Shell shock" was the term used during the First World War to describe psychological struggles faced by soldiers. During WWII, it was commonly referred to as "battle fatigue." It is now known as post-traumatic stress disorder.
Dr. Clarkson mentions that hospital beds will be needed for the injured from Arras. The Battle of Arras was a British offensive on the Western Front during World War I. The battle proved costly for both sides; British casualties numbered nearly 160,000 and the Germans about 125,000.
O'Brien refers to Thomas' injured hand as his "blighty." In this context, blighty is short for blighty wound, a minor wound serious enough to take a soldier out of combat.
The episode won an Emmy Award for Outstanding Hairstyling for a Single-Camera Series.
Iain Glenn is the second actor from Game of Thrones to appear in Downton Abbey after Rose Leslie. Glenn played the knight Ser Jorah Mormont, and Leslie played the wildling Ygritte.